Semiconductors form the basis of modern electronics. Possessing physical properties that can be selectively modified and controlled between conduction and insulation, semiconductors are essential in most modern electrical devices (e.g. computers, cellular phones, photovoltaic cells, etc.).
Typical solar cells are formed on a silicon substrate doped with a first dopant (the absorber region), upon which a second counter dopant is diffused using a gas or liquid process (the emitter region) completing the p-n junction. After the addition of passivation and antireflection coatings, metal contacts (fingers and busbar on the emitter and pads on the back of the absorber) may be added in order to extract generated charge carriers. Emitter dopant concentration, in particular, must be optimized for both carrier collection and for contact with the metal electrodes.
Electrons on the p-type side of the junction within the electric field (or built-in potential) tend to be attracted to the n-type region (usually doped with phosphorous) and repelled from the p-type region (usually doped with boron), whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region. Generally, the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration (e.g., phosphorous, arsenic, antimony, boron, aluminum, gallium, etc.) often shown as n−, n+, n++, p−, p+, p++, etc. The built-in potential and thus magnitude of electric field generally depend on the level of doping between the adjacent layers.
In some solar cell architectures, it may be beneficial to alter the type and concentration of a dopant as a function of substrate position. For example, for a selective emitter solar cell, a low concentration of (substitutional) dopant atoms within an emitter region will result in both low recombination (thus higher solar cell efficiencies), and poor electrical contact to metal electrodes. Conversely, a high concentration of (substitutional) dopant atoms will result in both high recombination (thus reducing solar cell efficiency), and low resistance ohmic contacts to metal electrodes. One solution, typically called a dual-doped or selective emitter, is generally to configure the solar cell substrate with a relatively high dopant concentration in the emitter region beneath the set of front metal contacts, and a relatively low dopant concentration in the emitter region not beneath the set of front metal contacts. Differential doping may also be beneficial to other solar cell architectures where the dopant needs to be localized, such as a backside contact solar cell.
Referring now to FIG. 1, a simplified diagram of a conventional solar cell is shown. In general, a moderately doped diffused emitter region 108 is generally formed above a relatively light and counter-doped diffused region absorber region 110. In addition, prior to the deposition of silicon nitride (SiNx) layer 104 on the front of the substrate, the set of metal contacts, comprising front-metal contact 102 and back surface field (BSF)/back metal contact 116, are formed on and fired into silicon substrate 110.
In a common configuration, a light n-type diffused region 108 (generally called the emitter or field), is formed by exposing the boron-doped substrate to POCl3 (phosphorus oxychloride) ambient to form phosphosilicate glass (PSG) on the surface of the wafer. The reduction of phosphorus pentoxide by silicon releases phosphorus into the bulk of the substrate and dopes it. The reaction is typically:4POCl3(g)+3O2(g)→2P2O5(l)+6Cl2(g)  [Equation 1A]2P2O5(l)+3Si(s)→5SiO2(s)+4P(s)  [Equation 1B]Si+O2→SiO2  [Equation 2]
The POCl3 ambient typically includes nitrogen gas (N2 gas) which is flowed through a bubbler filled with liquid POCl3, and a reactive oxygen gas (reactive O2 gas) configured to react with the vaporized POCl3 to form the deposition (processing) gas. In general, the reduction of P2O5 to free phosphorous is directly proportional to the availability of Si atoms.
Referring now to FIG. 2, a simplified diagram of a selective emitter is shown. In general, a relatively heavy n-type diffused region (high dopant concentration) 214 is generally formed in emitter areas beneath the set of front-metal contacts 202, while a relatively light n-type diffused region (low dopant concentration) 208 is generally formed in emitter areas not beneath the set of front-metal contacts 202. In addition, prior to the deposition of silicon nitride (SiNx) layer 204 on the front of the substrate, the set of metal contacts, comprising front-metal contact 202 and back surface field (BSF)/back metal contact 216, are formed on and fired into silicon substrate 210. In a common configuration, light n-type diffused region 208 (generally called the emitter or field), is formed by exposing the boron-doped substrate to POCl3 as previously described.
In an alternate configuration to those in FIGS. 1 & 2, the diffusion may be formed (or partially formed) using a doping paste directly deposited on the surface of the substrate, instead of through an ambient gas source. In general, an n-type or p-type dopant source is combined with some type of matrix material, preferably printable, that both provides the dopant source in a deposited pattern during the diffusion process, and is subsequently easily removed once the diffusion process has completed.
N-type doping pastes may include dopant precursors such as n-type liquids (i.e., phosphoric acid [H3PO4], organophosphates [O═P(OR)x(OH)3-x], etc.), n-type solids (i.e., P2O5, inorganic phosphates [Na3PO4, AlPO4, etc.] and phosphides [AlP, Na3P, etc.]), and n-type polymers (i.e., polyphosphonates, polyphosphazenes, etc.).
P-type doping pastes may include dopant precursors such as p-type liquids (i.e., borate esters [B(OR)3]), p-type solids (i.e., boric acid [B(OH)3], borates [NaBO2, Na2B4O7, B2O3]), p-type binary compounds (i.e., boronitride, boron carbide, boron silicides and elementary boron), and p-type polymers (i.e., polyborazoles, organoboron-silicon polymers, etc.).
An example of a common matrix material is a silica sol-gel. A “sol” is typically a stable suspension of colloidal particles within a liquid (2-200 nm), and a “gel” is a porous 3-dimensional interconnected solid network that expands in a stable fashion throughout a liquid medium and is limited by the size of the container.
In general, the sol-gel derived glass formation process involves first the hydrolysis of the alkoxide (sol formation), and second the polycondensation of hydroxyl groups (gelation). For a given silicon alkoxide of general formula Si(OR)4, R being an alkyl chain, these reactions can be written as follows:
HydrolysisSi(OR)4+H2O→(HO)Si(OR)3+R—OH  [Equation 3]
Condensation(HO)Si(OR)3+Si(OR)4→(RO)3Si—O—Si(OR)3+R—OH  [Equation 4](OR)3Si(OH)+(HO)Si(OR)3→(RO)3Si—O—Si(OR)3+H2O  [Equation 5]
For example, a sol-gel suspension (comprising silicon alkoxide) may be combined with an n-type precursor of phosphorus pentoxide (P2O5), like phosphoric acid (H3PO4), an organophosphate (O═P(OR)x(OH)3-x) etc. Likewise, p-type doping, the sol-gel suspension may be combined with a p-type precursor of boron trioxide (B2O3), like boric acid (B(OH)3), boron alkoxides (B(OR)3), etc. The resulting doped silicon glass (phosphoro-silicate glass (PSG) and boro-silicate glass (BSG) for n-type and p-type doping respectively) formed by condensation reaction during high temperature bake (200° C.<Tbake<500° C.) is used for subsequent dopant diffusion process.
However, the use of a sol-gel doping paste may be problematic for selective doping due to relatively low glass transition temperature of doped silicon glasses. Additionally, the glass transition temperature tends to decrease significantly with an increasing dopant contencentration corresponding to increasing atomic disorder of the silica layer. See J. W. Morris, Jr., Chapter 5: Glasses, Engineering 45 Notes, Fall 1995, UC Berkeley.
The glass transition temperature of doped silica glass formed from a typical doping paste is substantially below temperature needed to drive the dopant into the silicon substrate. As a result, the doped silica glass tends to reflow during high temperature processing resulting in spreading of the dopant source on the surface. While not problematic (and perhaps even beneficial) for the blanket doping of large substrates surfaces, the use of a doping process that produces a silicon glass is problematic for the forming of high-fidelity doping regions, such as would be required under the front metal fingers to form an ohmic contact.
In addition, many typical sol-gel doping pastes have sub-optimal screen printing characteristics. In general, in order to be commercially viable in high-volume solar cell production with a high printing resolution, a paste used in a screen printer must be a non-Newtonian shear-thinning fluid. Non-Newtonian fluid refers to a fluid whose flow properties are not described by a single constant value of viscosity. Shear thinning refers to a fluid whose viscosity decreases with increasing rate of shear stress.
Consequently, the viscosity of the paste must be relatively low at high shear rates in order to pass through a screen pattern, but must be relatively high prior to and after deposition (at low or zero shear rates), in order not to run through the screen or on the substrate surface respectively. However, many typical sol-gel doping pastes exhibit a near-Newtonian behavior, which means that they are either too viscous to effectively pass through a screen, or not viscous enough to prevent running, which corresponds to a low fidelity deposited pattern.
In view of the foregoing, there is desired a doping paste with a glass transition temperature substantially greater than the relevant doping temperature.